Radiation astronomy/Nebulas

This color picture was made by combining several exposures taken on the night of December 28th 1994 at the 0.9 m telescope of the Kitt Peak National Observatory. Credit: N.A.Sharp/NOAO/AURA/NSF.

"This color picture was made by combining several exposures taken on the night of December 28th 1994 (UT of observation 29/12/94 around 04:00) with a 2048x2048 CCD detector at the 0.9m telescope of the Kitt Peak National Observatory. Observing conditions were not ideal throughout, and so only a select few of the original observations were used. The final tally used five frames in the B (blue) filter for a total of 22 minutes, three frames with the V (green) filter, 15 minutes, and two with the R (red), total 10 minutes. Each frame was carefully cleaned, a particularly difficult task for the blue filter due to internal reflection problems in the telescope, and then aligned and combined by computer to create this (approximately) true color picture."[1]

"The Horsehead Nebula, a part of the optical nebula IC434 and also known as Barnard 33, was first recorded in 1888 on a photographic plate taken at the Harvard College Observatory. Its coincidental appearance as the profile of a horse's head and neck has led to its becoming one of the most familiar astronomical objects. It is, in fact, an extremely dense cloud projecting in front of the ionized gas that provides the pink glow so nicely revealed in this picture. We know this not only because the underside of the 'neck' is especially dark, but because it actually casts a shadow on the field to its east (below the 'muzzle')."[1]

This image shows an example of a bipolar planetary nebula known as PN Hb 12 in Cassiopeia. Credit: NASA, ESA, and A. Zijlstra (The University of Manchester).

"Hubble astronomers have found an unexpected surprise while surveying more than 100 planetary nebulae in the central bulge of our Milky Way galaxy. Those nebulae that are butterfly-shaped or hourglass-shaped tend to be mysteriously aligned such that their rotation axis is perpendicular to the plane of our galaxy."[3]

"Astronomers have used the NASA/ESA Hubble Space Telescope and ESO's New Technology Telescope to explore more than 100 planetary nebulae in the central bulge of our galaxy. They have found that butterfly-shaped members of this cosmic family tend to be mysteriously aligned — a surprising result given their different histories and varied properties."[4]

"Planetary nebulae are the expanding gaseous shrouds encircling dying stars. A subset of this population has bipolar outflows that align to the star's rotation axis. Such nebulae formed in different places and have different characteristics and so it is a puzzle why they should always point on the same sky direction, like bowling pins set up in an alley."[3]

"All these nebulae formed in different places and have different characteristics. Neither the individual nebulae, nor the stars that formed them, interact with other planetary nebulae. However, a new study by astronomers from the University of Manchester, UK, now shows surprising similarities between some of these nebulae: many of them line up in the sky in the same way. The "long axis" of a bipolar planetary nebula slices though the wings of the butterfly, whilst the "short axis" slices through the body."[4]

"The astronomers looked at 130 planetary nebulae in the Milky Way's central bulge. They identified three different types, and peered closely at their characteristics and appearance. The shapes of the planetary nebula images were classified into three types, following conventions: elliptical, either with or without an aligned internal structure, and bipolar."[4]

"This really is a surprising find and, if it holds true, a very important one, [...] Many of these ghostly butterflies appear to have their long axes aligned along the plane of our galaxy. By using images from both Hubble and the NTT we could get a really good view of these objects, so we could study them in great detail."[4]

"While two of these populations were completely randomly aligned in the sky, as expected, we found that the third — the bipolar nebulae — showed a surprising preference for a particular alignment, [...] While any alignment at all is a surprise, to have it in the crowded central region of the galaxy is even more unexpected."[3]

"Planetary nebulae are thought to be sculpted by the rotation of the star system from which they form. This is dependent on the properties of this system — for example, whether it is a binary [A binary system consists of two stars rotating around their common centre of gravity.], or has a number of planets orbiting it, both of which may greatly influence the form of the blown bubble. The shapes of bipolar nebulae are some of the most extreme, and are thought to be caused by jets blowing mass outwards from the star system perpendicular to its orbit."[3]

"The alignment we're seeing for these bipolar nebulae indicates something bizarre about star systems within the central bulge, [...] For them to line up in the way we see, the star systems that formed these nebulae would have to be rotating perpendicular to the interstellar clouds from which they formed, which is very strange."[4]

"While the properties of their progenitor stars do shape these nebulae, this new finding hints at another more mysterious factor. Along with these complex stellar characteristics are those of our Milky Way; the whole central bulge rotates around the galactic centre. This bulge may have a greater influence than previously thought over our entire galaxy — via its magnetic fields. The astronomers suggest that the orderly behaviour of the planetary nebulae could have been caused by the presence of strong magnetic fields as the bulge formed."[4]

"Researchers suggest that there is something bizarre about star systems within the central hub of our galaxy. They would all have to be rotating perpendicular to the interstellar clouds from which they formed. At present, the best guess is that the alignment is caused by strong magnetic fields that were present when the galactic bulge formed billions of years ago."[3]

"As such nebulae closer to home do not line up in the same orderly way, these fields would have to have been many times stronger than they are in our present-day neighbourhood. Very little is known about the origin and characteristics of the magnetic fields that were present in our galaxy when it was young, so it is unclear how they have changed over time."[4]

"We can learn a lot from studying these objects, [...] If they really behave in this unexpected way, it has consequences for not just the past of individual stars, but for the past of our whole galaxy."[3]

This is a wide-field image in the region of NGC 3603 taken on the ground by the Digitized Sky Survey 2. Credit: NASA, ESA, and the Digitized Sky Survey 2.

The large emission nebula NGC 6357 extends one degree on the sky in the direction of the Scorpius constellation. Credit: NASA, ESA and Jesœs Ma­z Apellÿniz (Instituto de astrof­sica de Andaluc­a, Spain), and Davide De Martin (ESA/Hubble).

This image of the Pistol Nebula was taken with an infrared spectrometer of the Hubble Telescope. Credit: Terry Herter, Ryan M. Lau, Mark Morris, and Joe Adams, NASA, Hubble, Cornell University.

Def. a "nebula composed of ionized gases which produce visible light"[5] is called an emission nebula.

"A wide-field image in the region of NGC 3603 [is] taken on the ground by the Digitized Sky Survey 2. The glowing clouds of hydrogen gas (seen here in orange) compose a vast emission nebula. The field of view is approximately 2.6 x 2.8 degrees."[6]

"The star cluster Pismis 24 lies in the core of the large emission nebula NGC 6357 [in the image on the right] that extends one degree on the sky in the direction of the Scorpius constellation. Part of the nebula is ionised by the youngest (bluest) heavy stars in Pismis 24. The intense ultraviolet radiation from the blazing stars heats the gas surrounding the cluster and creates a bubble in NGC 6357. The presence of these surrounding gas clouds makes probing into the region even harder."[7]

"Stretching across almost 100 light-years of space, the Snake nebula is located about 11,700 light-years from Earth in the direction of the constellation Ophiuchus."[8]

"In images from NASA's Spitzer Space Telescope, which observes infrared light, it appears as a sinuous, dark tendril against the starry background. It was targeted because it shows the potential to form many massive stars (stars with more than 8 times the mass of our Sun). SMA was used to observe sub-millimetre radiation from the nebula, radiation emitted between the infrared and radio parts of the electromagnetic spectrum."[8]

"The two panels [at right] show the Snake nebula as photographed by the Spitzer and Herschel space telescopes. At mid-infrared wavelengths (the upper panel taken by Spitzer), the thick nebular material blocks light from more distant stars. At far-infrared wavelengths, however (the lower panel taken by Herschel), the nebula glows due to emission from cold dust. The two boxed regions, P1 and P6, were examined in more detail by the Submillimeter Array."[8]

"To learn how stars form, we have to catch them in their earliest phases, while they're still deeply embedded in clouds of gas and dust, and the SMA is an excellent telescope to do so."[9]

"The team studied two specific spots within the Snake nebula, designated P1 and P6. Within those two regions they detected a total of 23 cosmic "seeds" -- faintly glowing spots that will eventually give birth to between one and a few stars. The seeds generally weigh between 5 and 25 times the mass of the Sun, and each spans a few hundred billion kilometres (for comparison the average Earth-Sun distance is 150 million km). The sensitive, high-resolution SMA images not only unveil the small seeds, but also differentiate them in age."[8]

"Previous theories proposed that high-mass stars form within very massive, isolated "cores" weighing at least 100 times the mass of the Sun. These new results show that that is not the case. The data also demonstrate that massive stars aren't born alone but in groups."[8]

The image second down on the left is a Hubble Telescope image using an infrared spectrometer. The Pistol "nebula of the insanely bright Pistol star is warped".[10]

"Dust in the Pistol star’s nebula is brilliant, compressed, externally heated and ionized, thanks to its proximity to neighbors in the Quintuplet Cluster."[11]

“The initial attention draw to the Pistol star was its high luminosity, [but] the nebulae around it and its sister star [LBV3] have turned out to be quite interesting. While the Pistol star is a member of the Quintuplet Cluster – although on the outskirts of the cluster – it is about six light-years away from the cluster’s center. That’s about 1.5 times the distance from our solar system to the nearest star.”[12]

“It’s impressive that even at this distance, the rest of the Quintuplet Cluster exerts a large influence on the Pistol nebula.”[12]

The dark lanes are actually made up of thick, opaque dust lying between us and the packed star field behind it. Credit: ESO.

"Rather than showing spectacular objects, some of the most surprising images of the Universe instead focus on emptiness. This new image [on the right] from the 2.2-metre MPG/ESO telescope shows dark tentacles swirling outwards from a dark, blank spot of space in the centre of the frame, particularly conspicuous against the dense peppering of bright gold and red stars across the rest of the image."[13]

"This region [in the constellation Ophiuchus] is not a hole in the cosmos, or an empty patch of sky. The dark lanes are actually made up of thick, opaque dust lying between us and the packed star field behind it. This obscuring dust forms part of a dark molecular cloud, cold and dense areas where large quantities of dust and molecular gas mingle and block the visible light emitted by more distant stars."[13]

"It is still unclear how these clouds form, but they are thought to be the very early stages of new star formation — in the future, the subject of this image may well collapse inwards on itself to form a new star system."[13]

"Although the cloud in this image is a fairly anonymous resident of the nearby Universe — catalogued as LDN1774 — one of the most famous examples of a molecular cloud is the very similar Barnard 68, which lies some 500 light-years away from us. Barnard 68 has been observed extensively using ESO telescopes, both in visible (eso9924a) and infrared light (eso9934, eso0102a). As shown in these different images, it is possible to probe through dark cosmic dust using infrared light, but visible-light observations such as those shown in this VLT image cannot see beyond the smokescreen."[13]

"This image was taken by the Wide Field Imager, an instrument mounted on ESO’s 2.2-metre MPG/ESO telescope at La Silla, Chile."[13]

"The 'streamers' visible in the brighter region appear to be due to a magnetic field which leaves the Horsehead cloud approximately radially, having been entrained by outflowing matter. Small red spots in the base of the Horsehead betray the presence of hidden protostars, and red streaks near the yellowish nebula surrounding V615 Orionis (bottom left) are Herbig-Haro objects, which are jets of material ejected from protostars. The Horsehead is a fascinating, active, and complex neighborhood."[1]

This image is a real color image of the planetary nebula NGC 6751 in Aquila. Credit: NASA/Hubble Space Telescope.

This dramatic new image of cosmic clouds in the constellation of Orion reveals what seems to be a fiery ribbon in the sky. Credit: ESO/Digitized Sky Survey 2.

"Astronomers using NASA's Hubble Space Telescope have obtained images of the strikingly unusual planetary nebula, NGC 6751. Glowing in the constellation Aquila like a giant eye, the nebula is a cloud of gas ejected several thousand years ago from the hot star visible in its center. The Hubble observations were obtained in 1998 with the Wide Field and Planetary Camera 2 (WFPC2) by a team of astronomers led by Arsen Hajian of the U.S. Naval Observatory in Washington, DC. The Hubble Heritage team, working at the Space Telescope Science Institute in Baltimore, has prepared this color rendition by combining the Hajian team's WFPC2 images taken through three different color filters that isolate nebular gases of different temperatures. The nebula shows several remarkable and poorly understood features. Blue regions mark the hottest glowing gas, which forms a roughly circular ring around the central stellar remnant. Orange and red show the locations of cooler gas. The cool gas tends to lie in long streamers pointing away from the central star, and in a surrounding, tattered-looking ring at the outer edge of the nebula. The origin of these cooler clouds within the nebula is still uncertain, but the streamers are clear evidence that their shapes are affected by radiation and stellar winds from the hot star at the center."[14]

"This dramatic new image of cosmic clouds in the constellation of Orion reveals what seems to be a fiery ribbon in the sky. The orange glow represents faint light coming from grains of cold interstellar dust, at wavelengths too long for human eyes to see. It was observed by the ESO-operated Atacama Pathfinder Experiment (APEX) in Chile."[15]

"In this image, the submillimetre-wavelength glow of the dust clouds is overlaid on a view of the region in the more familiar visible light, from the Digitized Sky Survey 2. The large bright cloud in the upper right of the image is the well-known Orion Nebula, also called Messier 42."[15]

The strange and irregular bundle of jets and clouds in this curious image from the NASA/ESA Hubble Space Telescope is the result of a burst of activity late in the life of a star. Credit: ESA/Hubble & NASA.

"The strange and irregular bundle of jets and clouds in this curious image from the NASA/ESA Hubble Space Telescope is the result of a burst of activity late in the life of a star. As its core runs out of nuclear fuel, the star’s unstable outer layers are puffing out a toxic concoction of gases including carbon monoxide and hydrogen cyanide."[17]

"The Westbrook Nebula — also known as PK166-06, CRL 618 and AFGL 618 — is a protoplanetary nebula, an opaque, dark and relatively short-lived cloud of gas that is ejected by a star as it runs out of nuclear fuel. As the star hidden deep in the centre of the nebula evolves further it will turn into a hot white dwarf and the gas around it will become a glowing planetary nebula, before eventually dispersing. Because this is a relatively brief stage in the evolution process of stars, only a few hundred protoplanetary nebulae are known in the Milky Way."[17]

"Protoplanetary nebulae are cool, and so emit little visible light. This makes them very faint, posing challenges to scientists who wish to study them. What this picture shows, therefore, is a composite image representing the different tricks that the astronomers used to unravel what is going on within this strange nebula. The picture includes exposures in visible light which shows light reflected from the cloud of gas, combined with other exposures in the near-infrared part of the spectrum, showing us the dim glow, invisible to human eyes, that is coming from different elements deep in the cloud itself."[17]

"One of the nebula’s names, AFGL 618, comes from its discovery by a precursor to the Hubble Space Telescope: the letters stand for Air Force Geophysics Laboratory. This US research organisation launched a series of suborbital rockets with infrared telescopes on board in the 1970s, cataloguing hundreds of objects that were impossible or difficult to observe from the ground. In some respects, these were a proof of concept for later orbital infrared astronomical facilities including Hubble and ESA’s Herschel Space Observatory."[17]

"This image was prepared from many separate exposures taken using Hubble’s newest camera, the Wide Field Camera 3. Exposures through a green filter (F547M) were coloured blue, those through a yellow/orange filter (F606W) were coloured green and exposures through a filter that isolates the glow from ionised nitrogen (F658N) have been coloured red. Images through filters that capture the glows from singly and doubly ionised sulphur (F673N and F953N) are also shown in red. The total exposure times were about nine minutes through each filter and the field of view is approximately 20 arcseconds across."[17]

ESO’s La Silla Observatory has snapped this new image of the famous Helix planetary nebula. Credit: Max-Planck Society/ESO telescope at the La Silla observatory in Chile.

"ESO’s La Silla Observatory has snapped a new image [at right] of the famous Helix planetary nebula, revealing a rich — and rarely photographed — background of distant galaxies."[18]

"The Helix Nebula, NGC 7293, about 700 light-years away in the constellation of Aquarius, is a Sun-like star in its final explosion before retirement as a white dwarf."[18]

"The blue-green glow in the centre of the Helix comes from oxygen atoms shining under effects of the intense ultraviolet radiation of the 120 000 degree Celsius central star and the hot gas. Further out from the star and beyond the ring of knots, the red colour from hydrogen and nitrogen is more prominent."[18]

"Shells of gas are blown off from the surface of such stars, often in intricate and beautiful patterns, and shine under the harsh ultraviolet radiation from the faint, hot central star. The main ring of the Helix Nebula is about two light-years across, or half the distance between the Sun and its nearest stellar neighbour."[18]

"Despite being photographically spectacular, the Helix is hard to see visually as its light is thinly spread over a large area of sky."[18]

This reflection nebula is associated with the bright star Rigel in the constellation Orion. Credit: Gary Stevens, NASA.

"This suggestively shaped reflection nebula [on the right] is associated with the bright star Rigel in the constellation Orion. More formally known as IC 2118, the Witch Head Nebula glows primarily by light reflected from Rigel, located just outside the top right corner of the above image. Fine dust in the nebula reflects the light. The blue color is caused not only by Rigel's blue color but because the dust grains reflect blue light more efficiently than red."[19]

"The object, aptly named the Necklace Nebula, is a recently discovered planetary nebula, the glowing remains of an ordinary, sun-like star."[20]

"The nebula consists of a bright ring, measuring 12trillion miles wide, dotted with dense, bright knots of gas that resemble diamonds in a necklace."[20]

"Newly discovered: The Necklace Nebula glows brightly in this composite image taken by the Hubble Space Telescope last month. The glow of hydrogen, oxygen, and nitrogen are shown by the colours blue, green and red respectively".[20]

"It is located 15,000 light-years away in the constellation Sagitta."[20]

"A pair of stars orbiting close together produced the nebula, also called PN G054.2-03.4."[20]

"About 10,000 years ago, one of the ageing stars ballooned to the point where it engulfed its companion star. The smaller star continued orbiting inside its larger companion, increasing the giant’s rotation rate. The bloated companion star spun so fast that a large part of its gaseous envelope expanded into space. Due to centrifugal force, most of the gas escaped along the star’s equator, producing a ring. The embedded bright knots are dense gas clumps in the ring. The pair is so close, only a few million miles apart, that they appear as one bright dot in the centre. The stars are furiously whirling around each other, completing an orbit in a little more than a day."[20]

This is the most detailed picture of IC 1295 object ever taken. Credit: ESO.

The Boomerang Nebula is a young planetary nebula and the coldest object found in the Universe so far. Credit: ESA/NASA.

This is an image of Boomerang nebula taken by Hubble Space Telescope. Credit: NASA, ESA and The Hubble Heritage Team (STScI/AURA).

This is an Atacama Large Millimeter/submillimeter Array (ALMA) telescope image of the Boomerang Nebula. Credit: NRAO/AUI/NSF/NASA/STScI/JPL-Caltech.

"This intriguing picture from ESO’s Very Large Telescope shows the glowing green planetary nebula IC 1295 [at right] surrounding a dim and dying star. It is located about 3300 light-years away in the constellation of Scutum (The Shield). This is the most detailed picture of this object ever taken."[21] Three filters are used in this image: the blue (B), visual (V) in green, and red (R) optical filters.[21] IC 1295 is at RA 18 54 37.25, Dec 39.41", the image is 6.82 x 6.82 arcminutes.[21]

"The Boomerang Nebula is a young planetary nebula and the coldest object found in the Universe so far. The NASA/ESA Hubble Space Telescope image is yet another example of how Hubble's sharp eye reveals surprising details in celestial objects."[22]

"NASA's Hubble Space Telescope caught the Boomerang Nebula [at second right] in images taken with the Advanced Camera for Surveys in early 2005. This reflecting cloud of dust and gas has two nearly symmetric lobes of matter that are being ejected from a central star. Each lobe of the nebula is nearly one light-year in length, making the total length of the nebula half as long as the distance from our Sun to our nearest neighbors- the Alpha Centauri stellar system, located roughly 4 light-years away. The Boomerang Nebula resides 5,000 light-years from Earth. Hubble's sharp view is able to resolve patterns and ripples in the nebula very close to the central star that are not visible from the ground."[23]

"This NASA/ESA Hubble Space Telescope image [at top right] shows a young planetary nebula known (rather curiously) as the Boomerang Nebula. It is in the constellation of Centaurus, 5000 light-years from Earth. Planetary nebulae form around a bright, central star when it expels gas in the last stages of its life."[22]

"The Boomerang Nebula is one of the Universe's peculiar places. In 1995, using the 15-metre Swedish ESO Submillimetre Telescope in Chile, astronomers Sahai and Nyman revealed that it is the coldest place in the Universe found so far. With a temperature of -272C, it is only 1 degree warmer than absolute zero (the lowest limit for all temperatures). Even the -270C background glow from the Big Bang is warmer than this nebula. It is the only object found so far that has a temperature lower than the background radiation."[22]

"The Hubble telescope took this image in 1998. It shows faint arcs and ghostly filaments embedded within the diffuse gas of the nebula's smooth 'bow tie' lobes. The diffuse bow-tie shape of this nebula makes it quite different from other observed planetary nebulae, which normally have lobes that look more like 'bubbles' blown in the gas. However, the Boomerang Nebula is so young that it may not have had time to develop these structures. Why planetary nebulae have so many different shapes is still a mystery."[22]

"The general bow-tie shape of the Boomerang appears to have been created by a very fierce 500 000 kilometre-per-hour wind blowing ultracold gas away from the dying central star. The star has been losing as much as one-thousandth of a solar mass of material per year for 1500 years. This is 10-100 times more than in other similar objects. The rapid expansion of the nebula has enabled it to become the coldest known region in the Universe."[22]

"The image was exposed for 1000 seconds through a green-yellow filter. The light in the image comes from starlight from the central star reflected by dust particles."[22] The image pixels have been coded blue even though the filter is centered at 606 nm.

"The Boomerang nebula, called the "coldest place in the universe," reveals its true shape to the Atacama Large Millimeter/submillimeter Array (ALMA) telescope. The background blue structure, as seen in visible light by NASA's Hubble Space Telescope, shows a classic double-lobe shape with a very narrow central region. ALMA’s resolution and ability to see the cold gas molecules reveals the nebula’s more elongated shape, as seen in red."[24]

NGC 3132 in Vela is a striking example of a planetary nebula. Credit: The Hubble Heritage Team (STScI/AURA/NASA).

"NGC 3132 [imaged at right] is a striking example of a planetary nebula. This expanding cloud of gas, surrounding a dying star, is known to amateur astronomers in the southern hemisphere as the "Eight-Burst" or the "Southern Ring" Nebula."[25]

"The name "planetary nebula" refers only to the round shape that many of these objects show when examined through a small visual telescope. In reality, these nebulae have little or nothing to do with planets, but are instead huge shells of gas ejected by stars as they near the ends of their lifetimes. NGC 3132 is nearly half a light year in diameter, and at a distance of about 2000 light years is one of the nearer known planetary nebulae. The gases are expanding away from the central star at a speed of 9 miles per second."[25]

"This image, captured by NASA's Hubble Space Telescope, clearly shows two stars near the center of the nebula, a bright white one, and an adjacent, fainter companion to its upper right. (A third, unrelated star lies near the edge of the nebula.) The faint partner is actually the star that has ejected the nebula. This star is now smaller than our own Sun, but extremely hot. The flood of ultraviolet radiation from its surface makes the surrounding gases glow through fluorescence. The brighter star is in an earlier stage of stellar evolution, but in the future it will probably eject its own planetary nebula."[25]

"In the Heritage Team's rendition of the Hubble image, the colors were chosen to represent the temperature of the gases. Blue represents the hottest gas [the oxygen 500.9 nm line], which is confined to the inner region of the nebula. Red represents the coolest gas [hydrogen Hα line], at the outer edge. The Hubble image also reveals a host of filaments, including one long one that resembles a waistband, made out of dust particles which have condensed out of the expanding gases. The dust particles are rich in elements such as carbon. Eons from now, these particles may be incorporated into new stars and planets when they form from interstellar gas and dust. Our own Sun may eject a similar planetary nebula some 6 billion years from now."[25]

The yellow line, or band, used as an intermediate temperature is due to the overlap between the oxygen cyan line and the Hα line.

The Red Rectangle is a proto-planetary nebula. Here is the Hubble Space Telescope Advanced Camera for Surveys (ACS) image. Broadband red light is shown in red. Credit: ESA/Hubble and NASA.

"[T]he extended red emission (ERE) [is] observed in many dusty astronomical environments, in particular, the diffuse interstellar medium of the Galaxy. ... silicon nanoparticles provide the best match to the spectrum and the efficiency requirement of the ERE."[26]

"The ERE was first recognized clearly in the peculiar reflection nebula called the Red Rectangle by Schmidt, Cohen, & Margon (1980)."[26]

The Red Rectangle Nebula, so called because of its red color and unique rectangular shape, is a protoplanetary nebula in the Monoceros constellation. Also known as HD 44179, the nebula was discovered in 1973 during a rocket flight associated with the AFCRL Infrared Sky Survey called Hi Star.

The "ERE manifests itself through a broad, featureless emission band of 60 < FWHM < 100 nm, with a peak appearing in the general wavelength range 610 < λp < 820 nm."[26]

In the Red Rectangle Nebula, diffraction-limited speckle images of it in visible and near infrared light reveal a highly symmetric, compact bipolar nebula with X-shaped spikes which imply toroidal dispersion of the circumstellar material. The central binary system is completely obscured, providing no direct light.[27]

"The star HD 44179 is surrounded by an extraordinary structure known as the Red Rectangle. It acquired its moniker because of its shape and its apparent colour when seen in early images from Earth. This strikingly detailed new Hubble image reveals how, when seen from space, the nebula, rather than being rectangular, is shaped like an X with additional complex structures of spaced lines of glowing gas, a little like the rungs of a ladder. The star at the centre is similar to the Sun, but at the end of its lifetime, pumping out gas and other material to make the nebula, and giving it the distinctive shape. It also appears that the star is a close binary that is surrounded by a dense torus of dust — both of which may help to explain the very curious shape. Precisely how the central engine of this remarkable and unique object spun the gossamer threads of nebulosity remains mysterious. It is likely that precessing jets of material played a role."[28]

"The Red Rectangle is an unusual example of what is known as a proto-planetary nebula. These are old stars, on their way to becoming planetary nebulae. Once the expulsion of mass is complete a very hot white dwarf star will remain and its brilliant ultraviolet radiation will cause the surrounding gas to glow. The Red Rectangle is found about 2 300 light-years away in the constellation Monoceros (the Unicorn)."[28]

"The High Resolution Channel of the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys captured this view of HD 44179 and the surrounding Red Rectangle nebula — the sharpest view so far. Red light from glowing Hydrogen was captured through the F658N filter and coloured red. Orange-red light over a wider range of wavelengths through a F625W filter was coloured blue."[28]

This is an image of planetary nebula NGC 7662, the Blue Snowball, in Andromeda. Credit: Adam Block, Caelum Observatory.

This is a color composite image of NGC 7662. Credit: Judy Schmidt.

At right are two images of the plasma associated with and a part of NGC 7662. The color of the nebula is very blue-green where the dominant light source is the 500.7 nm oxygen emission.

The second image is from the Hubble Space Telescope through three filters: F502N (blue), F555W (green), and F658N (red). The object is a planetary nebula (NGC 7662). A small star in the center has produced the nebula.

The two billowing structures in this NASA/ESA Hubble Space Telescope image of IRAS 13208-6020 are formed from material that is shed by a central star. Credit: ESA/Hubble & NASA.

"The two billowing structures in this NASA/ESA Hubble Space Telescope image of IRAS 13208-6020 are formed from material that is shed by a central star. This is a relatively short-lived phenomenon that gives astronomers an opportunity to watch the early stages of planetary nebula formation, hence the name protoplanetary, or preplanetary nebula. Planetary nebulae are unrelated to planets and the name arose because of the visual similarity between some planetary nebulae and the small discs of the outer planets in the Solar System when viewed through early telescopes."[29]

"This object has a very clear bipolar form, with two very similar outflows of material in opposite directions and a dusty ring around the star."[29]

"Protoplanetary nebulae do not shine, but are illuminated by light from the central star that is reflected back to us. But as the star continues to evolve, it becomes hot enough to emit strong ultraviolet radiation that can ionise the surrounding gas, making it glow as a spectacular planetary nebula. But before the nebula begins to shine, fierce winds of material ejected from the star will continue to shape the surrounding gas into intricate patterns that can only be truly appreciated later once the nebula begins to glow."[29]

"This picture was created from images taken using the High Resolution Channel of Hubble’s Advanced Camera for Surveys. Images taken through an orange filter (F606W, coloured blue) and a near-infrared filter (F814W, coloured red) have been combined to create this picture. The exposure times were 1130 s and 150 s respectively and the field of view is just 22 x 17 arcseconds."[29]

IC 5148 is a beautiful planetary nebula located some 3000 light-years away in the constellation of Grus (The Crane). Credit: ESO.

The hydrogen H-beta line (Hβ) has a wavelength of 486.1 nm.

On July 1, 1957, "Following the intense auroral display of the previous night, ... The variation in Hβ emission ... shows quite clearly that the sudden transition from an [auroral] arc to rays coincides with a decrease in the intensity of the hydrogen emission and an inversion of the polarity of the magnetic disturbance."[30]

"IC 5148 is a beautiful planetary nebula located some 3000 light-years away in the constellation of Grus (The Crane). The nebula has a diameter of a couple of light-years, and it is still growing at over 50 kilometres per second — one of the fastest expanding planetary nebulae known. The term “planetary nebula” arose in the 19th century, when the first observations of such objects — through the small telescopes available at the time — looked somewhat like giant planets. However, the true nature of planetary nebulae is quite different."[31]

"The ESO Faint Object Spectrograph and Camera (EFOSC2) on the New Technology Telescope at La Silla gives a somewhat more elegant view of this object. Rather than looking like a spare tyre, the nebula resembles ethereal blossom with layered petals."[31]

The familiar red H-alpha [Hα 656 nm] spectral line of hydrogen gas, which is the transition from the shell n = 3 to the Balmer series shell n = 2, is one of the conspicuous colors of the universe. It contributes a bright red line to the spectra of emission or ionization nebula, like the Orion Nebula, which are often H II regions found in star forming regions. In true-color pictures, these nebula have a distinctly pink color from the combination of visible Balmer lines that hydrogen emits.

The spectral lines from the atmospheres of spectral type O and B stars "show a large number of isolated and overlapping He I lines, the strongest of which are the spectral lines at 447.1 and 492.2 nm"[32].

"The Wide Field Camera 3 (WFC3), a new camera aboard NASA's Hubble Space Telescope, snapped this image of the planetary nebula, catalogued as NGC 6302, but more popularly called the Bug Nebula or the Butterfly Nebula. WFC3 was installed by NASA astronauts in May 2009, during the servicing mission to upgrade and repair the 19-year-old Hubble telescope."[33]

"What resemble dainty butterfly wings are actually roiling cauldrons of gas heated to more than 36,000 degrees Fahrenheit. The gas is tearing across space at more than 600,000 miles an hour—fast enough to travel from Earth to the Moon in 24 minutes!"[33]

"NGC 6302 lies within our Milky Way galaxy, roughly 3,800 light-years away in the constellation Scorpius. The glowing gas is the star's outer layers, expelled over about 2,200 years. The "butterfly" stretches for more than two light-years, which is about half the distance from the Sun to the nearest star, Alpha Centauri."[33]

"The central star itself cannot be seen, because it is hidden within a doughnut-shaped ring of dust, which appears as a dark band pinching the nebula in the center. The thick dust belt constricts the star's outflow, creating the classic "bipolar" or hourglass shape displayed by some planetary nebulae."[33]

"The star's surface temperature is estimated to be about 400,000 degrees Fahrenheit, making it one of the hottest known stars in our galaxy. Spectroscopic observations made with ground-based telescopes show that the gas is roughly 36,000 degrees Fahrenheit, which is unusually hot compared to a typical planetary nebula."[33]

"The WFC3 image reveals a complex history of ejections from the star. The star first evolved into a huge red-giant star, with a diameter of about 1,000 times that of our Sun. It then lost its extended outer layers. Some of this gas was cast off from its equator at a relatively slow speed, perhaps as low as 20,000 miles an hour, creating the doughnut-shaped ring. Other gas was ejected perpendicular to the ring at higher speeds, producing the elongated "wings" of the butterfly-shaped structure. Later, as the central star heated up, a much faster stellar wind, a stream of charged particles traveling at more than 2 million miles an hour, plowed through the existing wing-shaped structure, further modifying its shape."[33]

"The image also shows numerous finger-like projections pointing back to the star, which may mark denser blobs in the outflow that have resisted the pressure from the stellar wind."[33]

"The nebula's reddish outer edges are largely due to light emitted by nitrogen, which marks the coolest gas visible in the picture. WFC3 is equipped with a wide variety of filters that isolate light emitted by various chemical elements, allowing astronomers to infer properties of the nebular gas, such as its temperature, density, and composition."[33]

"The white-colored regions are areas where light is emitted by sulfur. These are regions where fast-moving gas overtakes and collides with slow-moving gas that left the star at an earlier time, producing shock waves in the gas (the bright white edges on the sides facing the central star). The white blob with the crisp edge at upper right is an example of one of those shock waves."[33]

"NGC 6302 was imaged on July 27, 2009, with Hubble's Wide Field Camera 3 in ultraviolet and visible light. Filters that isolate emissions from oxygen, helium, hydrogen, nitrogen, and sulfur from the planetary nebula were used to create this composite image."[33]

"M2-9 [in the image at right] is a striking example of a "butterfly" or a bipolar planetary nebula. Another more revealing name might be the "Twin Jet Nebula." If the nebula is sliced across the star, each side of it appears much like a pair of exhausts from jet engines. Indeed, because of the nebula's shape and the measured velocity of the gas, in excess of 200 miles per second, astronomers believe that the description as a super-super-sonic jet exhaust is quite apt. Ground-based studies have shown that the nebula's size increases with time, suggesting that the stellar outburst that formed the lobes occurred just 1,200 years ago."[34]

"The central star in M2-9 is known to be one of a very close pair which orbit one another at perilously close distances. It is even possible that one star is being engulfed by the other. Astronomers suspect the gravity of one star pulls weakly bound gas from the surface of the other and flings it into a thin, dense disk which surrounds both stars and extends well into space."[34]

"The disk can actually be seen in shorter exposure images obtained with the Hubble telescope. It measures approximately 10 times the diameter of Pluto's orbit. Models of the type that are used to design jet engines ("hydrodynamics") show that such a disk can successfully account for the jet-exhaust-like appearance of M2-9. The high-speed wind from one of the stars rams into the surrounding disk, which serves as a nozzle. The wind is deflected in a perpendicular direction and forms the pair of jets that we see in the nebula's image. This is much the same process that takes place in a jet engine: The burning and expanding gases are deflected by the engine walls through a nozzle to form long, collimated jets of hot air at high speeds."[34]

"M2-9 is 2,100 light-years away in the constellation Ophiucus. The observation was taken Aug. 2, 1997 by the Hubble telescope's Wide Field and Planetary Camera 2. In this image, neutral oxygen is shown in red, once-ionized nitrogen in green, and twice-ionized oxygen in blue."[34]

At right is an image gaseous objects ("cometary knots") discovered in the thousands. These knots are imaged with the Hubble Space Telescope while exploring the Helix nebula, the closest planetary nebula to Earth at 450 light-years away in the constellation Aquarius. Although ground-based telescopes have revealed such objects, astronomers have never seen so many of them. The most visible knots all lie along the inner edge of the doomed star's ring, trillions of miles away from the star's nucleus. Although these gaseous knots appear small, they're actually huge. Each gaseous head is at least twice the size of our solar system; each tail stretches for 100 billion miles, about 1,000 times the distance between the Earth and the Sun. The image was taken in August 1994 with Hubble's Wide Field Planetary Camera 2. The red light depicts nitrogen emission ([NII] 658.4 nm).

The second image at right is a color picture, taken with the Wide Field Planetary Camera-2. It is a composite of three images taken at different wavelengths. (red, hydrogen-alpha; blue, neutral oxygen, 630.0 nm; green, ionized nitrogen, 658.4 nm). This NASA Hubble Space Telescope image shows one of the most complex planetary nebulae ever seen, NGC 6543, nicknamed the "Cat's Eye Nebula." The image was taken on September 18, 1994. NGC 6543 is 3,000 light-years away in the northern constellation Draco. The term planetary nebula is a misnomer; dying stars create these cocoons when they lose outer layers of gas.

"The breathtaking butterfly-like planetary nebula NGC 6881 is visible here [at right] in an image taken by the NASA/ESA Hubble Space Telescope. Located in the constellation of Cygnus, it is formed of an inner nebula, estimated to be about one fifth of a light-year across, and symmetrical “wings” that spread out about one light-year from one tip to the other. The symmetry could be due to a binary star at the nebula’s centre."[35]

"NGC 6881 has a dying star at its core which is about 60% of the mass of the Sun. It is an example of a quadrupolar planetary nebula, made from two pairs of bipolar lobes pointing in different directions, and consisting of four pairs of flat rings. There are also three rings in the centre."[35]

"A planetary nebula is a cloud of ionised gas, emitting light of various colours. It typically forms when a dying star — a red giant — throws off its outer layers, because of pulsations and strong stellar winds."[35]

"The image was taken through three filters which isolate the specific wavelength of light emitted by nitrogen (N II, 658 nm, shown in red), hydrogen (Hα, 656 nm, shown in green) and oxygen (O III, 502 nm, shown in blue)."[35]

Several red astronomy emission lines are detected and recorded at normalized intensities (to the oxygen III line) from the Ring Nebula. In the red are the two forbidden lines of oxygen ([O I], 630.0 and 636.4 nm), two forbidden lines of nitrogen ([N II], 654.8 nm and [N II], 658.4 nm), the hydrogen line (Hα, 656.3 nm) and a forbidden line of sulfur ([S II], 671.7 nm).

The HIFI spectrum of the Orion Nebula is superimposed on a Spitzer image of Orion. Credit: ESA, HEXOS and the HIFI Consortium.

"The HIFI spectrum of the Orion Nebula, superimposed on a Spitzer image of Orion. A characteristic feature is the spectral richness: among the organic molecules identified in this spectrum are water, carbon monoxide, formaldehyde, methanol, dimethyl ether, hydrogen cyanide, sulfur oxide, sulfur dioxide and their isotope analogues. It is expected that new molecules will also be identified. This spectrum is the first glimpse at the spectral richness of regions of star and planet formation. It harbors the promise of a deep understanding of the chemistry of space once the complete spectral surveys are available."[36]

"This HIFI spectrum was obtained for the Herschel HEXOS Key Program - a scientific investigation using the Herschel HIFI and PACS instruments to perform full line surveys of five sources in the Orion and Sagittarius B2 molecular clouds."[36]

"The broad, 60 < FWHM < 100 nm, featureless luminescence band known as extended red emission (ERE) is seen in such diverse dusty astrophysical environments as reflection nebulae17, planetary nebulae3, HII regions (Orion)12, a Nova11, Galactic cirrus14, a dark nebula7, Galaxies8,6 and the diffuse interstellar medium (ISM)4. The band is confined between 540-950 nm, but the wavelength of peak emission varies from environment to environment, even within a given object. ... the wavelength of peak emission is longer and the efficiency of the luminescence is lower, the harder and denser the illuminating radiation field is13. These general characteristics of ERE constrain the photoluminescence (PL) band and efficiency for laboratory analysis of dust analog materials."[37]

"The PL efficiencies measured for HAC and Si-HAC alloys are consistent with dust estimates for reflection nebulae and planetary nebulae, but exhibit substantial photoluminescence below 540 nm which is not observed in astrophysical environments."[37]

Various layers of material are expelled by the central star. Credit: ESA/Hubble and NASA.

"It may look like something from "The Lord of the Rings," but this fiery swirl is actually a planetary nebula known as ESO 456-67. Set against a backdrop of bright stars, the rust-colored object lies in the constellation of Sagittarius (The Archer), in the southern sky."[38]

"In this image of ESO 456-67, it is possible to see the various layers of material expelled by the central star. Each appears in a different hue - red, orange, yellow, and green-tinted bands of gas are visible, with clear patches of space at the heart of the nebula. It is not fully understood how planetary nebulae form such a wide variety of shapes and structures; some appear to be spherical, some elliptical, others shoot material in waves from their polar regions, some look like hourglasses or figures of eight, and others resemble large, messy stellar explosions - to name but a few."[38]

Shock waves through icy parts of the solar nebula may be the mechanism that enriched ancient meteorites. Credit: William K. Hartmann, Planetary Science Institute, Tucson, Arizona.

Def. a "disc-shaped cloud of gas and dust left over from the formation of the Sun"[39] is called a solar nebula.

In the artist's impression on the right: "Shock waves through icy parts of the solar nebula may be the mechanism that enriched ancient meteorites (called chondrites) with water -- water that some believe provided an otherwise dry Earth with oceans."[40]

An HI cloud apparently is near or directly in front of the quasar 3C 147. Credit: Hubble Legacy Archive.

The bright areas of this image of the LMC are where the most atomic hydrogen gas is found. Credit: S. Kim et al. / CSIRO.

Def. an interstellar cloud composed primarily of neutral atomic hydrogen is called an HI cloud, H I cloud, or HI region.

"Although there is a possibility that we are seeing the edge of a larger feature, we may be seeing a cloud of higher density superposed on a slowly varying background. If one assumes that to be the case, one finds that the H I cloud has a column density 1020 atoms cm-2 at maximum (assuming an arbitrary kinetic temperature of 50 K and a half-width of 2 km s-1). Although one cannot determine the distance to the absorbing cloud, one can estimate a reasonable upper limit. The quasar 3C 247 [in the image on the right] lies at galactic latitude 100; the assumption of a hydrogen layer extending 100 pc above the plane leads to a maximum probable distance of 600 pc. The linear diameter of the cloud (if the angular diameter is taken to be 0.1") is then at most 3 x 10-4 pc, or 70 AU! The neutral hydrogen density is 105 atoms cm-3; the mass, 3 x 10-7M⊙."[42]

Galaxies "around us are hiding about a third more atomic hydrogen gas than previously calculated."[43]

The neutral atomic hydrogen "gas is distributed very differently from how it was in the past, with much less in the galaxies’ outer suburbs than billions of years ago."[44]

“This means that it’s much harder for galaxies to pull the gas in and form new stars. It’s why stars are forming 20 times more slowly now than in the past.”[44]

“Even though there’s more atomic hydrogen than we thought, it’s not a big enough percentage to solve the Dark Matter problem. If what we are missing had the weight of a large kangaroo, what we have found would have the weight of a small echidna.”[44]

"The Southern Galactic Plane Survey (SGPS; see the 2002 Annual Report), which combines 21-cm HI observations from Parkes and the Compact Array, is now complete. The SGPS provides a wonderful resource for understanding populations such as magnetars in the context of their environment. Examination of SGPS data around the position of the well-known magnetar 1E 1048.1­5937 reveals a striking cavity in HI, designated as GSH 288.3-0.5-28, that is almost centred on the position of the neutron star. The SGPS data imply that GSH 288.3-0.5-28 is at a distance of approximately 2.7 kpc, and is expanding at a velocity of approximately 7.5 kilometres per second into gas of density ~17 atoms cm-3."[45]

"Shells like GSH 288.3-0.5-28 are common, and represent wind-blown bubbles powered by massive stars expanding into the interstellar medium. The size and expansion speed of GSH 288.3-0.5-28 then imply that the bubble is several million years old, and has been blown by a wind of mechanical luminosity ~4 x 1034 ergs per second, corresponding to a single star of initial mass 30 to 40 solar masses."[45]

"Usually in such cases, the central star is obvious, in the form of a bright O star, supergiant or WR star at the shell's centre. However, even though this field lies in the rich Carina OB1 region, there are no known stars of the appropriate position, distance or luminosity to argue for an association with GSH 288.3-0.5-28. This raises the intriguing possibility that GSH 288.3-0.5-28 was blown by the massive star whose collapse formed 1E 1048.1-5937. The central location of the magnetar within the HI shell suggests that the supernova occurred quite recently. The corresponding blast waves would impact the walls of the HI shell approximately 3000 years after core collapse, producing significant X-ray and radio emission. The lack of such emission requires the neutron star to be very young, consistent with the small ages expected for active magnetars. A common distance of around three kpc is suggested by the properties of both objects."[45]

In the upper image on the right, the reddish region is a giant HII cloud.

Def. an interstellar cloud in which the primary constituent is monatomic hydrogen undergoing ionization and emission is called an HII cloud.

"The nebula [in the second image down on the right] is mostly composed of hydrogen gas, which is ionised by the ultraviolet radiation emitted by the hot stars, leading to the nebula’s alternative title as an HII region. This picture shows only part of the nebula, where dark dust clouds are strikingly silhouetted against the glowing gas."[46]

"NGC 2174 lies about 6400 light-years away in the constellation of Orion (The Hunter)."[46]

"This picture was created from images from the Wide Field Planetary Camera 2 on Hubble. Images through four different filters were combined to make the view shown here. Images through a filter isolating the glow from ionised oxygen (F502N) were coloured blue and images through a filter showing glowing hydrogen (F656N) are green. Glowing ionised sulphur (F673N) and the view through a near-infrared filter (F814W) are both coloured red. The total exposure times per filter were 2600 s, 2600 s, 2600 s and 1000 s respectively and the field of view is about 1.8 arcminutes across."[46]

"The Maryland-Green Bank hydrogen-line survey maps reveal this feature [the emission nebula surrounding NGC 2175] as part of a large neutral hydrogen cloud in the galactic plane that is situated at the edge of the association Gem.I. It is most unlikely that such a large neutral hydrogen cloud would be connected with the emission nebula surrounding NGC 2175. Indeed, in a medium with a mean density of hydrogen atoms of 20 cm-3, the Strömgren radius of an HII region around an O6-type star would be more than 16 pc.* However, if a distance of 2 kpc is accepted, the linear radius of the full extent of the continuum source is less than 10 pc. Thus the ionized nebula is density bounded rather than ionization bounded, its small size implying that it is not part of a large neutral hydrogen cloud which would be ionized by radiation from the O6-type star."[47]

N 164 is a bright nebula, the glow of which is caused by hot stars inside it. Credit: ESO.

"N 164, a bright nebula, the glow of which is caused by hot stars inside it. The heating of the gas by these stars increases the pressure and causes such nebulae to expand, pushing outwards against their surroundings. A careful look at this nebula reveals locations where the expansion is encountering resistance by denser clouds of gas, producing bright, thin rims. The sky field measures 3.6 x 3.5 arcmin. North is up and East is left."[48]

Def. an interstellar cloud that glows from the hot stars inside it is called a bright nebula.

This picture shows Barnard 59, part of a vast dark cloud of interstellar dust called the Pipe Nebula. Credit: ESO.

Def. a "type of nebula that unlike other types of nebulae does not emit or reflect light and therefore appears as a starless region in the sky"[49] is called a dark nebula.

"This picture [on the right] shows Barnard 59, part of a vast dark cloud of interstellar dust called the Pipe Nebula. This new and very detailed image of what is known as a dark nebula was captured by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory. This image is so large that it is strongly recommended to use the zoomable version to appreciate it fully."[50]

The Pipe Nebula is around 600 lyrs distant in the constellation Ophiuchus.[50]

Herbig-Haro object HH 47 is a bipolar stellar jet of 4.83 trillion kilometers long and 10 times the width of our Solar System. Credit: J. Morse/STScI, and NASA/ESA.

Def. "a small, bright knot of nebular emission in a dark, interstellar cloud of gas and dust"[51] is called a Herbig-Haro object.

"Herbig-Haro object HH 47 [imaged on the right] is a bipolar stellar jet of 4.83 trillion kilometers long and 10 times the width of our Solar System located at the edge of the Gum Nebula, about 1140 light-years away in the southern constellation of Vela."[52]

"Herbig–Haro objects (HH) – named after astronomers George Herbig and Guillermo Haro – are narrow jets of gas and matter ejected by young stars at speeds of 100 to 1000 kilometers per second that collide with clouds of gas and dust nearby. They are ubiquitous in star-forming regions, and several are often seen around a single star, aligned along its rotational axis. The stellar jets seem to form as the swirling cloud of dust and gas surrounding a new star escapes."[52]

"These objects are transient phenomena, lasting not more than a few thousand years. They can evolve visibly over quite short timescales as they move rapidly away from their parent star into the gas clouds in interstellar space."[52]

"HH 47’s central low mass protostar – which contains water and carbon dioxide ices, as well as organic molecules – is ejecting a jet and creates a bipolar outflow, and lies inside a Bok globule (a dark nebula which contain very young stars)."[52]

"This image clearly reveals a very complicated jet pattern that indicates the star (hidden inside a dust cloud near the left edge of the image) might be wobbling, possibly caused by the gravitational pull of a companion star. The jet has burrowed a cavity through the dense gas cloud and now travels at high speed into interstellar space."[52]

"Shock waves form when the jet collides with interstellar gas, causing the jet to glow. The white filaments on the left bottom reflect light from the obscured newborn star."[52]

"This VLT image of the Thor’s Helmet Nebula [on the right] was taken on the occasion of ESO’s 50th Anniversary, 5 October 2012, with the help of Brigitte Bailleul — winner of the Tweet Your Way to the VLT! competition. The observations were broadcast live over the internet from the Paranal Observatory in Chile. This object, also known as NGC 2359, lies in the constellation of Canis Major (The Great Dog). The helmet-shaped nebula is around 15 000 light-years away from Earth and is over 30 light-years across. The helmet is a cosmic bubble, blown as the wind from the bright, massive star near the bubble's centre sweeps through the surrounding molecular cloud."[57]

"This wind-blown bubble [NGC 2359] in the interstellar medium has been shaped by the outflow from a massive star near its its center. The ultraviolet light from the same star causes the nebula to glow."[58]

Def. a core of a dark cloud, having supersonic and subsonic turbulence,[62] is called a dense core.

"30 Doradus is the brightest star-forming region in our galactic neighbourhood and home to the most massive stars ever seen. The nebula resides 170 000 light-years away in the Large Magellanic Cloud, a small, satellite galaxy of our Milky Way. No known star-forming region in our galaxy is as large or as prolific as 30 Doradus."[63]

"The image comprises one of the largest mosaics ever assembled from Hubble photos and includes observations taken by Hubble’s Wide Field Camera 3 and Advanced Camera for Surveys, combined with observations from the European Southern Observatory’s MPG/ESO 2.2-metre telescope which trace the location of glowing hydrogen and oxygen."[63]

The dense core R136 is about 170,000 lyrs away in the constellation Dorado.[63]

"Although VY Can is about half a million times as luminous as the Sun, much of its visible light is absorbed by a large, asymmetric cloud of dust particles that has been ejected from the star in various outbursts over the past 1,000 years or so. The infrared emission from this dust cloud makes VY Can one of the brightest objects in the sky at wavelengths of 5–20 microns."[65]

"In 2007, a team of astronomers using the 10-meter radio dish on Mount Graham, in Arizona, found that VY Can's extended circumstellar cloud is a prolific molecule-making factory. Among the radio emissions identified were those of hydrogen cyanide (HCN), silicon monoxide (SiO), sodium chloride (NaCl) and a molecule, phosphorus nitride (PN), in which a phosphorus atom and a nitrogen atom are bound together. Phosphorus-bearing molecules are of particular interest to astrobiologists because phosphorus is relatively rare in the universe, yet it is a key ingredient in molecules that are central to life as we know it, including the nuclei acids DNA and RNA and the energy-storage molecule, ATP. "[65]

"Material ejected by the star is visible in this 2004 image [on the top right] captured by the Hubble Space Telescope's Advanced Camera for Surveys, using polarizing filters."[65]

For comparison, the second image down on the right is captured using visuals.

Smith's Cloud is a hydrogen gas, high-velocity cloud on the outskirts of the Milky Way Galaxy. Credit: Bill Saxton, NRAO/AUI/NSF.

Def. any cloud having a velocity "inconsistent with simple Galactic rotation models that generally fit the stars and gas in the Milky Way disk" is called a high-velocity cloud.[66]

"The leading edge of this cloud [shown in the image on the right] is already interacting with gas from our Galaxy."[67]

"The cloud, called Smith's Cloud, after the astronomer who discovered it in 1963, contains enough hydrogen to make a million stars like the Sun. Eleven thousand light-years long and 2,500 light-years wide, it is only 8,000 light-years from our Galaxy's disk. It is careening toward our Galaxy at more than 150 miles per second, aimed to strike the Milky Way's disk at an angle of about 45 degrees."[68]

"This is most likely a gas cloud left over from the formation of the Milky Way or gas stripped from a neighbor galaxy. When it hits, it could set off a tremendous burst of star formation. Many of those stars will be very massive, rushing through their lives quickly and exploding as supernovae. Over a few million years, it'll look like a celestial New Year's celebration, with huge firecrackers going off in that region of the Galaxy."[67]

"If you could see this cloud with your eyes, it would be a very impressive sight in the night sky. From tip to tail it would cover almost as much sky as the Orion constellation. But as far as we know it is made entirely of gas -- no one has found a single star in it."[67]

"Its shape, somewhat similar to that of a comet, indicates that it's already hitting gas in our Galaxy's outskirts. It is also feeling a tidal force from the gravity of the Milky Way and may be in the process of being torn apart. Our Galaxy will get a rain of gas from this cloud, then in about 20 to 40 million years, the cloud's core will smash into the Milky Way's plane."[67]

Def. a "stage in stellar evolution when a star starts to shed its outer layers before coming a true planetary nebula"[69] is called a preplanetary nebula.

In the image on the right: "What resemble dainty butterfly wings are actually roiling cauldrons of gas heated to more than 36,000 degrees Fahrenheit. The gas is tearing across space at more than 600,000 miles an hour -- fast enough to travel from Earth to the moon in 24 minutes!"[70]

"A dying star that was once about five times the mass of the Sun is at the center of this fury. It has ejected its envelope of gases and is now unleashing a stream of ultraviolet radiation that is making the cast-off material glow. This object is an example of a planetary nebula, so-named because many of them have a round appearance resembling that of a planet when viewed through a small telescope."[70]

"The Wide Field Camera 3 (WFC3), a new camera aboard NASA’s Hubble Space Telescope, snapped this image of the planetary nebula, catalogued as NGC 6302, but more popularly called the Bug Nebula or the Butterfly Nebula. WFC3 was installed by NASA astronauts in May 2009, during the servicing mission to upgrade and repair the 19-year-old Hubble telescope."[70]

"NGC 6302 lies within our Milky Way galaxy, roughly 3,800 light-years away in the constellation Scorpius. The glowing gas is the star’s outer layers, expelled over about 2,200 years. The "butterfly" stretches for more than two light-years, which is about half the distance from the Sun to the nearest star, Alpha Centauri."[70]

"The central star itself cannot be seen, because it is hidden within a doughnut-shaped ring of dust, which appears as a dark band pinching the nebula in the center. The thick dust belt constricts the star’s outflow, creating the classic "bipolar" or hourglass shape displayed by some planetary nebulae."[70]

"The star’s surface temperature is estimated to be about 400,000 degrees Fahrenheit, making it one of the hottest known stars in our galaxy. Spectroscopic observations made with ground-based telescopes show that the gas is roughly 36,000 degrees Fahrenheit, which is unusually hot compared to a typical planetary nebulae."[70]

"The WFC3 image reveals a complex history of ejections from the star. The star first evolved into a huge red-giant star, with a diameter of about 1,000 times that of our Sun. It then lost its extended outer layers. Some of this gas was cast off from its equator at a relatively slow speed, perhaps as low as 20,000 miles an hour, creating the doughnut-shaped ring. Other gas was ejected perpendicular to the ring at higher speeds, producing the elongated "wings" of the butterfly-shaped structure. Later, as the central star heated up, a much faster stellar wind, a stream of charged particles travelling at more than 2 million miles an hour, plowed through the existing wing-shaped structure, further modifying its shape."[70]

'The image also shows numerous finger-like projections pointing back to the star, which may mark denser blobs in the outflow that have resisted the pressure from the stellar wind."[70]

"The nebula's outer edges are largely due to light emitted by nitrogen, which marks the coolest gas visible in the picture. WFC3 is equipped with a wide variety of filters that isolate light emitted by various chemical elements, allowing astronomers to infer properties of the nebular gas, such as its temperature, density, and composition."[70]

"The white-colored regions are areas where light is emitted by sulfur. These are regions where fast-moving gas overtakes and collides with slow-moving gas that left the star at an earlier time, producing shock waves in the gas (the bright white edges on the sides facing the central star). The white blob with the crisp edge at upper right is an example of one of those shock waves."[70]

"NGC 6302 was imaged on July 27, 2009 with Hubble’s Wide Field Camera 3 in ultraviolet and visible light. Filters that isolate emissions from oxygen, helium, hydrogen, nitrogen, and sulfur from the planetary nebula were used to create this composite image."[70]

By contrast, on the left is the preplanetary (protoplanetary) nebula IRAS 09371+1212, nicknamed the Frosty Leo Nebula.

"Despite their name, protoplanetary nebulae have nothing to do with planets: they are formed from material shed from their aging central star. The Frosty Leo Nebula has acquired its curious name as it has been found to be rich in water in the form of ice grains, and because it lies in the constellation of Leo."[71]

"This nebula is particularly noteworthy because it has formed far from the galactic plane, away from interstellar clouds that may block our view. The intricate shape comprises a spherical halo, a disc around the central star, lobes and gigantic loops. This complex structure strongly suggests that the formation processes are complex and it has been suggested that there could be a second star, currently unseen, contributing to the shaping of the nebula."[71]

The gaseous outer layers of a Sun-like star glow in space after being expelled as the star reached the end of its life. Credit: NASA, ESA, and the Hubble Heritage Team.

The planetary nebula Messier 57, also known as the Ring Nebula, in the constellation Lyra, exhibits cyan coloration surrounding its central region. Credit: The Hubble Heritage Team (AURA/STScI/NASA).

Def. a "nebulosity surrounding a dying star, consisting of material expelled by the star"[72] is called a planetary nebula.

NGC 7048 is a planetary nebula in the constellation of Cygnus. The bright star to the lower left of the nebula is a magnitude 10.5 star. The nebula is slightly brighter along the west and east sides. This planetary nebula is rated at magnitude 12.1. NGC 7048 was discovered by Jean Marie Edouard Stephan on October 1878 using a 31.5-inch reflector.

In recent years, Hubble Space Telescope images have revealed many planetary nebulae to have extremely complex and varied morphologies. About a fifth are roughly spherical, but the majority are not spherically symmetric.

"The Hubble Space Telescope has imaged striking details of the famed planetary nebula designated NGC 2818 [at left], which lies in the southern constellation of Pyxis (the Compass). The spectacular structure of the planetary nebula contains the outer layers of a star that were expelled into interstellar space. The glowing gaseous shrouds in the nebula were shed by the central star after it ran out of fuel to sustain the nuclear reactions in its core."[74]

"This Hubble image was taken in November 2008 with the Wide Field Planetary Camera 2. The colors in the image represent a range of emissions coming from the clouds of the nebula: red represents nitrogen, green represents hydrogen, and blue represents oxygen."[74]

Successive photos of V838 Monocerotis show the progress of a light echo. Credit: NASA, ESA, H.E. Bond (STScI) and The Hubble Heritage Team (STScI/AURA).

"Gas around V838 Monocerotis, nova-like star, seemed to be expanding faster than light from the earth."[75]

"This image [on the right] shows a time sequence of Hubble Space Telescope images of the light echo around V838 Mon, taken between May 2002 and [February] 2004. All [five] pictures were taken with Hubble's Advanced Camera for Surveys using filters sensitive to blue, visible, and infrared wavelengths. The apparent expansion of the light echo, as light from the early 2002 outburst of V838 Mon propagates outward into the surrounding dust".[76]

"All of the images are shown at the same scale. Moreover, the images are also shown as they would appear for the same exposure times throughout the sequence. Thus the background stars appear constant in brightness, while the surface brightness of the light echo steadily declines. The fading of the light echo is primarily due to the light-scattering properties of interstellar dust. Consider a street lamp on a foggy night. The halo around the lamp is brightest right next to the lamp, while out to the side it is much fainter. Similarly, in the first V838 Mon image, taken in May 2002, the light echo was very bright and compact. At later times, we are seeing dust out to the side of the star, rather than dust that is immediately in front of the star, so the amount of light scattered in our direction is smaller. Hubble astronomers expect the light echo to continue to change its appearance and brightness over the next several years."[76]

Def. a "nebula that is formed from supernova explosion debris"[78] is called a supernova remnant.

The image on the right is a color "composite of the supernova remnant E0102-72: X-ray (blue), optical (green), and radio (red). E0102-72 is the remnant of a star that exploded in a nearby galaxy known as the Small Magellanic Cloud. The galaxy is approximately 190,000 light years from Earth, so we see the remnant as it was about 190,000 years ago, around a thousand years after the explosion occurred."[79]

"The star exploded outward at speeds in excess of 20 million kilometers per hr (12 million mph) and collided with surrounding gas. This collision produced two shock waves, or cosmic sonic booms one traveling outward, and the other rebounding back into the material ejected by the explosion."[79]

"The radio image was made using the Australia Telescope Compact Array. The radio waves are due to extremely high-energy electrons spiraling around magnetic field lines in the gas and trace the outward moving shock wave."[79]

"The Chandra X-ray image, shown in blue, shows gas that has been heated to millions of degrees Celsius by the rebounding, or reverse shock wave. The X-ray data show that this gas is rich in oxygen and neon. These elements were created by nuclear reactions inside the star and hurled into space by the supernova."[79]

"The Hubble Space Telescope optical image shows dense clumps of oxygen gas that have "cooled" to about 30,000 degree Celsius."[79]

E0102-72.3 is in the constellation Tucana at RA 01h 04m 02.40s | Dec -72° 01' 55.30, and the image is 1 arcmin across.[79]

Def. a "nebula powered by the pulsar wind of a pulsar"[80] is called a plerion.

"Detailed studies with the Hubble Space Telescope [captured in the image on the right] illustrate the rich structure of the Crab Nebula's synchrotron emission, on scales down to 0.2 arcseconds. These are also evident in the X-ray image [...]. Especilly intriguing are the structures of the inner nebula near the pulsar--wisps, knots, and fibrous texture--which exhibit cylindrical symmetry. HST observations also show that wisps form and dissipate over a few weeks while moving outward at about 0.5c."[81]

The NASA/ESA Hubble Space Telescope has used its powerful optics to separate the globular cluster NGC 6401 into its constituent stars. Credit: ESA/Hubble & NASA.

"The NASA/ESA Hubble Space Telescope has used its powerful optics to separate the globular cluster NGC 6401 into its constituent stars. What was once only visible as a ghostly mist in the eyepieces of astronomical instruments has been transformed into a stunning stellar landscape."[82]

"NGC 6401 can be found within the constellation of Ophiuchus (The Serpent Bearer). The globular cluster itself is relatively faint, so a telescope and some observational experience are required to see it. Globular clusters are very rich, and generally spherical, collections of stars, hence the name. They orbit the cores of galaxies, with the force of gravity also keeping the stars bound as a group. There are around 160 globular clusters associated with our Milky Way, of which NGC 6401 is one. These objects are very old, containing some of the most ancient stars known. However, there are many mysteries surrounding them, with the origin of globular clusters and their role within galaxy evolution not being completely understood."[82]

"The famous astronomer William Herschel discovered this cluster in 1784 with his 47 cm telescope, but mistakenly believed it to be a bright nebula. Later his son, John Herschel, was to make the same error — evidently the technology of the day was insufficient to allow the individual stars to be resolved visually."[82]

"NGC 6401 has confused more modern astronomers as well. In 1977 it was thought that a low-mass star in the cluster had been discovered venting its outer layers (known as a planetary nebula). However, a further study in 1990 concluded that the object is in fact a symbiotic star: a binary composed of a red giant and a small hot star such as a white dwarf, with surrounding nebulosity. It could be that the study in 1977 was simply a few thousand years ahead of its time, as symbiotic stars are thought to become a type of planetary nebula."[82]

"This picture was created from images taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Images through a yellow-orange filter (F606W, coloured blue) were combined with images taken in the near-infrared (F814W, coloured red). The total exposure times were 680 s and 580 s, respectively and the field of view is 3.3 x 1.5 arcminutes."[82]

This picture was put together from images taken with the Wide Field Channel of Hubble's Advanced Camera for Surveys. Credit: ESA/Hubble & NASA.

"The dazzling stars in Messier 15 [at right] look fresh and new in this image from the NASA/Hubble Space Telescope, but they are actually all roughly 13 billion years old, making them some of the most ancient objects in the Universe. Unlike another recent Hubble Picture of the Week, which featured the unusually sparse cluster Palomar 1, Messier 15 is rich and bright despite its age."[83]

"Messier 15 is a globular cluster — a spherical conglomeration of old stars that formed together from the same cloud of gas, found in the outer reaches of the Milky Way in a region known as the halo and orbiting the Galactic Centre. This globular lies about 35 000 light-years from the Earth, in the constellation of Pegasus (The Flying Horse)."[83]

"Messier 15 is one of the densest globulars known, with the vast majority of the cluster’s mass concentrated in the core. Astronomers think that particularly dense globulars, like this one, underwent a process called core collapse, in which gravitational interactions between stars led to many members of the cluster migrating towards the centre."[83]

"Messier 15 is also the first globular cluster known to harbour a planetary nebula, and it is still one of only four globulars known to do so. The planetary nebula, called Pease 1, can be seen in this image as a small blue blob to the lower left of the globular’s core."[83]

"This picture was put together from images taken with the Wide Field Channel of Hubble's Advanced Camera for Surveys. Images through yellow/orange (F606W, coloured blue) and near-infrared (F814W, coloured red) filters were combined. The total exposure times were 535 s and 615 s respectively and the field of view is 3.4 arcminutes across."[83]

"The gas in the clouds of NGC 6559, mainly hydrogen, is the raw material for star formation ... When a region inside this nebula gathers enough matter, it starts to collapse under its own gravity. The center of the cloud grows ever denser and hotter, until thermonuclear fusion begins and a star is born. The hydrogen atoms combine to form helium atoms, releasing energy that makes the star shine. ... In regions where it is very dense, the dust completely blocks the light behind it, as is the case for the dark isolated patches and sinuous lanes to the bottom left-hand side and right-hand side of the image".[84]

"The Danish 1.54-metre telescope located at ESO’s La Silla Observatory in Chile has captured a striking image of NGC 6559, an object that showcases the anarchy that reigns when stars form inside an interstellar cloud. This region of sky includes glowing red clouds of mostly hydrogen gas, blue regions where starlight is being reflected from tiny particles of dust and also dark regions where the dust is thick and opaque."[85]

"The two colors of the cloud represent a pair of nebulas. Once the young stars are born, they "energize" the hydrogen surrounding them, ESO officials said. The gas then creates the red wispy cloud — known to astronomers as an "emission nebula" — in the center of the image."[86]

"These young stars are usually of spectral type O and B, with temperatures between 10 000 and 60 000 K, which radiate huge amounts of high energy ultraviolet light that ionises the hydrogen atoms."[87]

"The blue section of the photo — representing a "reflection nebula" — shows light from the newly formed stars in the cosmic nursery being reflected in all directions by the particles of dust made of iron, carbon, silicon and other elements in the interstellar cloud."[86]

NGC 6559 is planetary nebula located at a distance of about 5000 light-years from Earth, in the constellation of Sagittarius.

"NGC 6559 is a cloud of gas and dust located at a distance of about 5000 light-years from Earth, in the constellation of Sagittarius (The Archer). The glowing region is a relatively small object, just a few light-years across, in contrast to the one hundred light-years and more spanned by its famous neighbour, the Lagoon Nebula (Messier 8, eso0936). Although it is usually overlooked in favour of its distinguished companion, NGC 6559 has the leading role in this new picture."[87]

"The Milky Way fills the background of the image with countless yellowish older stars. Some of them appear fainter and redder because of the dust in NGC 6559."[87]

"Observations made with the APEX telescope in submillimetre-wavelength light at a wavelength of 870 µm reveal the cold dusty clouds from which stars form in the Carina Nebula. This site of violent star formation, which plays host to some of the highest-mass stars in our galaxy, is an ideal arena in which to study the interactions between these young stars and their parent molecular clouds."[88]

"The APEX observations, made with its LABOCA camera, are shown here in orange tones, combined with a visible light image from the Curtis Schmidt telescope at the Cerro Tololo Interamerican Observatory. The result is a dramatic, wide-field picture that provides a spectacular view of Carina’s star formation sites. The nebula contains stars equivalent to over 25 000 Suns, and the total mass of gas and dust clouds is that of about 140 000 Suns."[88]

"The marked change in the density of stars visible on either side [of the Horsehead Nebula at page top] indicates that the strip of glowing hydrogen marks the edge of a substantial dark cloud. As a cloud core emerging from its parental cloud, and as an active site of low-mass star formation, the Horsehead is a simple system of considerable use for testing models of photodissociation regions, and revealing the intricate interrelations between gas, dust, and the light from hot stars. Polarization maps suggest that the entire region is illuminated by the bright OB star Sigma Orionis, which is also responsible for exciting the emission nebula. (The much brighter Zeta Orionis is a foreground star, not related to the nebulosity.)"[1]

This planetary nebula is known as Kohoutek 4-55 (or K 4-55). Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).

The recent history period dates from around 1,000 b2k to present.

"The Hubble community bids farewell to the soon-to-be decommissioned Wide Field Planetary Camera 2 (WFPC2) onboard the Hubble Space Telescope. In tribute to Hubble's longest-running optical camera, a planetary nebula [at right] has been imaged as WFPC2's final "pretty picture.""[89]

"This planetary nebula is known as Kohoutek 4-55 (or K 4-55). It is one of a series of planetary nebulae that were named after their discoverer, Czech astronomer Lubos Kohoutek. A planetary nebula contains the outer layers of a red giant star that were expelled into interstellar space when the star was in the late stages of its life. Ultraviolet radiation emitted from the remaining hot core of the star ionizes the ejected gas shells, causing them to glow."[89]

"In the specific case of K 4-55, a bright inner ring is surrounded by a bipolar structure. The entire system is then surrounded by a faint red halo, seen in the emission by nitrogen gas. This multi-shell structure is fairly uncommon in planetary nebulae."[89]

"This Hubble image was taken by WFPC2 on May 4, 2009. The colors represent the makeup of the various emission clouds in the nebula: red represents nitrogen, green represents hydrogen, and blue represents oxygen. K 4-55 is nearly 4,600 light-years away in the constellation Cygnus."[89]

"The WFPC2 instrument, which was installed in 1993 to replace the original Wide Field/Planetary Camera, will be removed to make room for Wide Field Camera 3 during the upcoming Hubble Servicing Mission."[89]

"During the camera's amazing, nearly 16-year run, WFPC2 provided outstanding science and spectacular images of the cosmos. Some of its best-remembered images are of the Eagle Nebula pillars, Comet P/Shoemaker-Levy 9's impacts on Jupiter's atmosphere, and the 1995 Hubble Deep Field — the longest and deepest Hubble optical image of its time."[89]

"The scientific and inspirational legacy of WFPC2 will be felt by astronomers and the public alike, for as long as the story of the Hubble Space Telescope is told."[89]